The invention relates to an improved method and apparatus for bracing a cross-arm on a support pole, in particular on a utility cable-bearing support pole.
In above-ground public utility transmission systems, such as telephone or electric power transmission systems for example, electrical currents are generally transmitted by means of cables suspended between utility poles. The poles are often of a circular cross-section and each includes one or more cross-arms to which cables are mounted by means of insulators. A cross-arm can be attached to a pole by various means, such as by a clamp, a gain fixture, or, most often, by a bolt running diametrically through both the cross-arm and the pole.
Under circumstances such as high winds after icing precipitation, the suspended cables can begin to oscillate. This oscillation puts a strain on the cross-arms and tends to rotate them about an axis formed by the support pole. This back-and-forth rotating motion in turn fatigues the bolts used to connect the cross-arms to the poles, causing the cross-arm to break away from the support pole. This in turn generally causes the cables to contact the support poles, thereby creating danger of fires started by sparks, interruption of service, and the burdensome cost, in terms of both money and time, of repairing or replacing the cables, cross-arms and, often, the poles.
The prior art has attempted to solve this problem by the use of grid gains. A grid gain is a metal plate, generally about the size of the cross-section of the cross-arm, with one of its faces contoured to fit the surface of the pole and the other face contoured to receive the cross-arm. The gain is permanently inserted between the cross-arm and a pole before the two are attached together. Thus the gain essentially provides a surface area for the cross-arm to contact, as opposed to only a line or a point area of contact when the cross-arm is mounted directly to the pole, and thereby the gain offers some resistance to the rotation of the cross-arm.
While useful, the gain has not completely solved the problem. First, the gain can be expensive to manufacture, depending upon its complexity. Second, if the gain were made large enough to solve the rotation problem, a different size of gain would be required for each range of shapes and sizes of poles and cross-arms, since the gain must be contoured to fit their surfaces. Third, in retrofit installations, the cross-arm must be dismounted from the pole so that the gain may be inserted between them, which results in high costs in terms of effort and money and often also results in interruption of service during retrofitting. Fourth, the gain, in its currently available sizes, has not been totally effective in preventing the rotation of the cross-arm because the gain provides a support surface area for the cross-arm only in the immediate area surrounding the axis of rotation, where the support is least effective.
Another approach known to the industry to reduce cross-arm rotation has been a back brace, generally consisting of two rods, attached to and extending between the back side of the pole and some point along the length of the cross-arm, with one rod extending to each end of the cross-arm. But this approach suffers from certain disadvantages as well. When a back brace is retrofitted, the cross-arm may have to be dismounted and re-installed and the cross-arm must be field drilled in two places for the purpose of attaching the brace thereto. Consequently, this approach has been found to be both difficult and costly to implement.
Yet another problem plaguing the utility industry has been the fires on poles started when precipitation has wetted the insulators, cross-arms, and poles, thus substantially decreasing their surface resistivity and causing them to act as conductors for power line leakage current. Under certain conditions of precipitation, the areas around the bolt connecting the pole and cross-arm and the surface between the pole and cross-arm remain dry, leaving a high resistance gap between the wetted conductive surfaces of the pole and cross-arm, which gap effectively acts as an electric heater with the flow of the electric current. Resulting heating and arcing can ignite fires which can destroy the entire utility pole.